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Howdy all,
I am looking to use my shimano dura ace 7700 GS (long cage) rear derailleur with a 9 speed mtn cassette with 34 tooth max cog. I would like to use this cassette for touring because of it's incredible low gearing. Is the 34 tooth to big for this derailleur? thanks for everyones help.

When I was investigating a cassette range bump (12-25 to 12-32), my LBS informed me that I would need a proper mtb derailleur to be able function properly with such a large cog. But that was just my experience, and I ended up going with an XT shadow RD to ensure functionality.

I've gotten my Dura-Ace RD to run on a 30 but it barely clears that. I doubt that you'll be able to get it to run on a 34. I believe the spec for max cog is 27 regardless if it short or long cage. You should look at getting a Shimano MTB rear derailer.